One lens for beginner semi-pro?

Hi all I am looking for some advice please. I am starting off part time photography work having done some family shoots, studio head shots and pregnancy photos with speedlights/studio setting, and lots of community events. I am still learning with every shoot but my clients have been happy with the results. I have been using equipment loaned from my employer but need to become independent and get my own gear.
I have:
Canon EOS 7D body only
Occasional access to canon 50mm 1.8 and canon 18-200mm superzoom
580exII flash and couple of cheap manual flashes for speedlight work
Light stands / couple of softboxes and pixel wireless flash trigger

BIG Question: What is the ONE lens that I should get to start me off. My budget is not huge - US $800. I am guessing that my money making work will be more family groups / headshots / possible wedding work. At present the canon 18-135 seems to around the price range, with a good range of length to use in a variety of settings... the 15-85 and 24-105 canons are also popular but are getting too pricey

Any recommendations (with some reasoning if possible) would be greatly appreciated
Cheers!